Anti-surge control for fluid compressor



July 2, 1963 w. CLARK 3,095,710

ANTISURGE CONTROL FOR FLUID COMPRESSOR Filed May 18, 1960 INVENTOR.

WILLIAM E. CLARK ATTORNEY.

United States Patent Ofiice 3,095,710 ANTI-SURGE CONTROL FOR FLUID COMPRESSOR William E. Clark, Syracuse, N.Y., assignor to Carrier Corporation, Syracuse, N.Y., a corporation of Delaware Filed May 18, 1960, Ser. No. 29,962 3 Claims. (Cl. 62-117) This invention relates broadly to the control of fluid compression equipment. More particularly, this invention relates to a control for use with fluid compression equipment subject to surging action. Still more particularly, this invention rel-ates to a control for equipment of the type described wherein provision is made for the novel detection of an approaching surge of a magnitude sufficient to cause damage to the equipment.

In fluid processing apparatus wherein a fluid or gas compressor is employed as a component such as a refrigeration machine, the gas compressor, which may be a centrifugal compressor or an axial compressor, is employed for the purpose of circulating fluid from the low pressure side of the apparatus to the high pressure side of the apparatus.

In apparatus of this type, very often a variation in an apparatus operating condition will cause a relatively low volume of low pressure gas at the inlet of the compressor. Inasmuch as the compressor normally is designed so as to develop a given head at a given speed, it will be appreciated that the discharge pressure from the compressor likewise is reduced. Pressure downstream of the compressor discharge, however, is at a relatively greater pressure and under circumstances where the discharge pressure developed by the compressor is insuflicient to assure continued flow of the relatively high pressure gas, reverse flow of the high pressure gas occurs to the extent that flow of gas actually occurs from the discharge to the suction side of the apparatus through the compressor.

Various devices have been proposed for the purpose of detecting a conditionin the machine which indicates that a surging action will ensue. This inventionhas for its chief object the provision of a control for this purpose which is novel and which includes a minimum of operating parts so as to be relatively inexpensive in construction.

A further object of this invention is the provision of a control of the kind described where-in the criterion employed for the purpose of sensing approaching surge is related ot the superheat present in the gas at the inlet to the compressor.

A still further object of the invention is the provision of a control device for regulating surge in a compressor employed in a refrigeration system using a thermal expansion valve for controlling flow of refrigerant from the condenser to the evaporator.

In achieving the objects of the invention there is provided a gas processing apparatus including a fluid compressor, means for bypassing a portion of the gas discharged from the compressor to the suction of the compressor and a control for flow through said bypass responsive to. predetermined difference in the temperature of the gas at the inlet to the compressor and an established datum in the processing apparatus.

These and other objects of the invention will be apparent upon a consideration of the ensuing specification and drawing in which the FIGURE is a diagrammatic representation of fluid processing apparatus such as a refrigeration system employing a compressor susceptible to surge which is equipped wtih a control of the type contemplated by this invention. j

Referring morev particularly to the drawing for an illustration of the invention it will be apparent that the 3,095,710 Patented July 2, 1963 fluid processing plant chosen to illustrate the invention is a refrigeration system having a turbo-compressor 10 including a steam or air propelled turbine 12 as a prime mover and a compressor wheel 14 of the centrifugal type mounted on a common shaft.

The steam or air under pressure used to operate the turbine is delivered through supply line 16 having a suitable control for regulating the amount of fluid supplied to the turbine. The compressor 14 receives gaseous refrigerant, compresses it and delivers it through discharge line 18 to condenser 20 where it is liquefied when passed in heat transfer relation with a suitable cooling medium.

Liquid refrigerant formed in the condenser flows through line 24 to a receiver 22 having a liquid line connection 26 to evaporator 28. Interposed in line 26 is a thermal expansion valve 30 of the well known type wherein liquid refrigerant is metered to the evaporator in accordance with a particular degree of superheat present in the gaseous refrigerant flowing from the evaporator. To this end, bulb 31 fonning part of a thermal responsive system is connected to the valve 30 by a capillary 33.

Gaseous refrigerant formed in the evaporator flows into a vessel 32 serving as a mixing chamber and mounting for bulb 31 in a manner to be hereinafter described. Extending upwardly from the bottom of the vessel for the purpose of receiving gaseous refrigerant is the open end of suction line 34. The other end of the suction line connects to the inlet of the compressor 14 to complete the refrigerant flow cycle.

As indicated above, with a compressor of the kind described, a reduction in the loading on the system may result in a supply of gaseous refrigerant at the entrance to the compressor of a volume insufficient to maintain high pressure refrigerant flow in the discharge line upon delivery from the compressor. For the purpose of augmenting the volume of relatively low pressure gas present at the inlet of the compressor under these conditions, a bypass line 36 is employed to serve as a path of flow for a portion of the high pressure refrigerant present in the receiver through the mixing chamber ot the suction line 34. Flow in the bypass line is under the control of valve 38. Valve 38 controlling flow of gaseous refrigerant in the bypass line 36 is provided with a capillary 39 and a bulb 40, housing, with a portion 41 above diaphragm 42 of valve 38, a thermal responsive fill in the same manner as valve 30. The bulb 40 is employed to sense the temperature of the gas at the inlet to the compressor. The under surface of diaphragm 42 is subjected to the pressure in the low prmsure side of the system through connection 44. Thus valve 38 operates in response to a differential pressure created -by the difference in the temperature of the gas at the inlet to the effect of surge activity in a gas compressor of the kind under consideration here, it has been thought that as the machine approaches the operating region of unstable act-ion known as surge, there is a relatively small volume of localized reverse flow that occurs along the surfaces of the conduit leading into the compressor prior to an actual reversal of total or substantially totall flow. Thus,

.it has been fairly well established that for a relatively short period of time flow of gas in the suction line actually occurs in two directions simultaneously. The relatively small portion of gas flow occurring along the surfaces of the conduit continues rearwardly and then joins manner.

the gas flow entering the machine. This flow which incidently is of relatively high temperature having had part of the heat of compression added thereto, raises the temperature of bulb 40 positioned in the suction line adjacent the inlet to the compressor after it flows backward through the compressor.

The relatively small amount of high temperature gas leaking rearwardly from the compressor 14 in the manner described is of relatively high pressure. The amount of this gas is too small to be a significant factor in changing the force applied to the underside of diaphragm 42 through line 44. In this regard, the low pressure side, particularly the chamber 32, acts in the manner of a surge tank, dampening the high pressure pulsation. In a practical sense, therefore, a small amount of surge usually referred to as incipient surge, is used to regulate the operation of the valve controlling flow in the bypass line. Once the valve in the bypass line opens, a volume of gas i made available in the mixing chamber for supply along with the relatively low pressure, low temperature, gas from the evaporator to the compressor. This action provides a supply of refrigerant gas of a suflicient vdlume so as to enable the compressor to operate in its stable range and thereby maintain continuity of how downstream of the compressor and prevent continual surging of a magnitude that would be harmful to the compressor. Thus this invention uses as a control for the bypass line a factor indicative of ensuing surge which possess surge characteristics of a minor order that are not particularly harmful to the compressor.

The mixture of the gas from the bypass line with the refrigerant gas in the vessel serving as a mixing chamber raises the temperature sensed by the bulb of the thermal expansion valve controlling the passage of the fliquid from the condenser'to the evaporator. The effect of the rise in temperature is to open the valve and permit passage of more liquid refrigerant to the extent that possibly some liquid refrigerant enters the mixing chamber. The amount involved, however, is not of a great enough extent to impair the action of the compressor particularly in view of the arrangement wherein the suction line is mounted with its open end spaced from the bottom of the vessel, causing the vessel to act in the manner of an accumulator.

Considering the operation of the apparatus described the compressor is operative under control of the turbine to extract gaseous refrigerant from the evaporator and forward it to the condenser in the usual conventional Under those circumstances where the load on the evaporator is decreased the amount of refrigerant circulated through the evaporator is reduced by the action of the thermal expansion valve as it equates refrigerant feed to evaporator demand. In the event the load continues to decrease, thermal expansion valve 30 will eventually attain a closed position. As the load decreases, or a higher discharge pressure is required, the machine approaches the range of unstable operation and the locaflized reverse flow described above occurs. The control for valve 38 senses the increase in superheat at the compressor inlet and acts to open the valve and bypass a portion of the discharge of the compressor to the suction of the compressor. Valve 38 throttles the flow of relatively high pressure refrigerant into the mixing chamber where it is mixed with the relatively small volume of low pressure gas. Under these circumstances, both the volume and pressure are increased so that the gas suppiied to the compressor when compressed by the compressor is of a pressure sufficient to assure a continuous flow of high pressure gas through the high pressure side of the system.

The novel control heretofore described is useful particularly in systems employing a variable speed prime mover such as the turbine described above. Normally, the control for such a system attempts to equate the speed of the turbine drive with the pumping requirement under certain loading conditions. When the load decreases the turbine speed is reduced further impairing development of a head or pressure ratio sufficiently large enough to assure compressor discharge pressures sufficient to prevent the tendency :for reverse flow attendant the surging action. With the control described, the bypass line is operative to supply a gas requirement to the suction of the compressor to assure suitable operation of the compressor despite turbine speed.

Another advantage of the invention resides in the utilization of a mixing chamber arranged so that gaseous refrigerant from the evaporator and from the bypass line are each tangentially fed into the chamber to provide an efiicient mixture of gas. The bulb controlling the thermal expansion valve metering refrigerant to the evaporator is arranged so as to sense the superheat in the mixture of gases as they flow into the suction line for eventual supply to the compressor. It is not necessary that the bulb be so p lacedfor it has been found that positioning the bulb at the evaporator exit in the usual manner may be desirable. Also desuperheated gaseous refrigerantfrom the condenser or any other location in the system may be used for how in line 36.

While I have described a preferred embodiment of the invention, it will be understood the invention is not limited thereto since it may be otherwise embodied within the scope of the following claims.

I claim:

1. The method of controlling operation of a compressor subject to surging action in a refrigeration system including in addition to the compressor a condenser for liquefying refrigerant, an evaporator for vaporizing refrigerant and means for metering refrigerant flow to said evaporator, which consists in the steps of providing a path of flow for refrigerant from the high pressure side of the system to the low pressure side of the system without flow through said metering means, controlling flow of refrigerant-in said path in response to a predetermined difference in the temperature of the gaseous refrigerant at the inlet of the compression means and the saturated temperature of the gaseous refrigerant in the evaporator as determined by a measured characteristic of the refrigerant indicative of the saturatedtemperature of the refrigerant in the evaporator.

2. In a refrigeration system including an evaporator, a condenser, a thermal expansion valve metering flow of refrigerant from the condenser to the evaporator, compression means subject to surging action for receiving gaseous refrigerant from the evaporator for supply to the condenser, a bypass line connecting the high pressure side of the system with the low pressure side of the system for the purpose of supplying relatively high pressure refrigerant to the inlet of the compression means to elevate the suction pressure and assure the development of sufficient discharge pressure by the compression means to resist reverse flow of refrigerant through the compression means, means controlling flow in said bypass line, said last mentioned meansrincluding a thermal responsive control operable in response to apredetermined difference in the temperature of the gaseous refrigerant at the inlet of the compression means and the saturated temperature of the gaseous refrigerant in the evaporator as determined by a measured characteristic of the refrigerant indicative of the saturated temperature of the refrigerant in the evaporator.

3. The invention set forth in claim 2 wherein said measured characteristic is refrigerant pressure.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF CONTROLLING OPERATION OF A COMPRESSOR SUBJECT TO SURGING ACTION IN A REFRIGERATION SYSTEM INCLUDING IN ADDITION TO THE COMPRESSOR A CONDENSER FOR LIQUEFYING REFRIGERANT, AN EVAPORATOR FOR VAPORIZING REFRIGERANT AND MEANS FOR METERING REFRIGERANT FLOW TO SAID EVAPORATOR, WHICH CONSISTS IN THE STEPS OF PROVIDING A PATH OF FLOW FOR REFRIGERANT FROM THE HIGH PRESSURE SIDE OF THE SYSTEM TO THE LOW PRESSURE SIDE OF THE SYSTEM WITHOUT FLOW THROUGH SAID METERING MEANS, CONTROLLING FLOW OF REFRIGERANT IN SAID PATH IN RESPONSE TO A PREDETERMINED DIFFERENCE IN THE TEMPERATURE OF THE GASEOUS REFRIGERANT AT THE INLET OF THE COMPRESSION MEANS AND THE SATURATED TEMPERATURE OF THE GASEOUS REFRIGERANT IN THE EVAPORATOR AS DETERMINED BY A MEASURED CHARACTERISTIC OF THE REFRIGERANT INDICATIVE OF THE SATURATED TEMPERATURE OF THE REFRIGERANT IN THE EVAPORATOR. 